@mmangiamusardo please mark your post as having spoiler content next time.

for input {2,2,1,3,2,2,3,1,1,2,2,3,3,2,1,3} my algorithm produces valid solution {{1,3,4,2},{4,2,1,3},{3,4,2,1},{2,1,3,4}}
yet the test says it's wrong.

Enjoyable coding challenge, congratulations.

For what it's worth, from my perspective I thought it was pretty clear. I wonder what you think is poorly explained?

Had to make a web app so that I could visualize & interact with the problem, but after that I was able to find a solution. Very fun kata, not like any I had done before, +1

Well now I feel silly.

I don't think the problem is well explained at all.

it is very likely that one hand get used multiple times

So, that's it! Thanks, now totally explained.

In all of the initial tests Hands are created, tested once and then freed. That let me to the assumption that it would always be that way. My mistake. Thanks again.

yes, but what is the issue here ? you modify tracker yourself, so you mutate the state of your Hand object. then at the next call with the same object, of course the mutations will persist. take a look at how the random tests work, all of the hands are built beforehand and are played against each other at random, it is very likely that one hand get used multiple times, hence why the changes persist

Thanks, but that result would be expected - buildTracker() should indeed set values into the tracker field.

Let's use your test approach, but before any calls to buildTracker:

Result compare(Hand *pl, Hand *op) {
  int checksum = (pl->tracker == 0) + (op->tracker == 0);
  // Both tracker values were set to 0 by PokerHand, so they should still be 0 now.
  assert(checksum == 0);

  // tracker fields will be updated by buildTracker().
  pl->type = buildTracker(pl, pl->cards);
  op->type = buildTracker(op, op->cards);
  Result retval = _compare(pl, op);
  return retval;
}

Output includes:

boxfort-worker: solution.c:199: Result compare(Hand *, Hand *): Assertion `checksum == 0' failed.

it seems to be your own code that modifies the tracker field though, I slightly modified your test in compare():

Result compare(Hand *pl, Hand *op) {
  int checksum = (pl->tracker == 0) + (op->tracker == 0);
  pl->type = buildTracker(pl, pl->cards);
  op->type = buildTracker(op, op->cards);
  Result retval = _compare(pl, op);
  int new_checksum = (pl->tracker == 0) + (op->tracker == 0);
  assert(checksum == new_checksum);
  return retval;
}

the assertion fails.

cool))

This code does not really confirm anything because we do not know how your real code, the one which breaks things, looks like.